1. Field of the Invention
This invention relates to an improved process for increasing gasoline octane number in catalytic cracking units without decreasing total liquid yield. This invention particularly relates to increasing gasoline octane without a significant loss in C.sub.6.sup.+ fraction, i.e., gasoline plus distillate, yield with the use of ZSM-5 type catalysts, which have been selectivated (as defined hereinafter) by exposure to high temperature partial pressure steam, in admixture with cracking catalysts.
2. Description of the Prior Art
Hydrocarbon conversion processes utilizing crystalline zeolites have been the subject of extensive investigation during recent years, as is obvious from both the patent and scientific literature. Crystalline zeolites have been found to be particularly effective for a wide variety of hydrocarbon conversion processes, including the catalytic cracking of a gas oil to produce motor fuels, and have been described and claimed in many patents, including U.S. Pat. Nos. 3,140,249; 3,140,251; 3,140,252; 3,140,253; and 3,271,418. It is also known in the prior art to incorporate the crystalline zeolite into a matrix for catalytic cracking, and such disclosure appears in one or more of the above-identified U.S. patents.
It is also known that improved results will be obtained with regard to the catalytic cracking of gas oils if a crystalline zeolite having a pore size of less than 7 angstrom units is admixed with a crystalline zeolite having a pore size greater than 8 angstrom units, either with or without a matrix. A disclosure of this type is found in U.S. Pat. No. 3,769,202. Although the incorporation of a crystalline zeolite having a pore size of less than 7 angstrom units into a catalyst comprising a larger pore size crystalline zeolite (pore size greater than 8 angstrom units) has indeed been very effective with respect to the raising of octane number, nevertheless it did so at the expense of the yield of gasoline.
Improved octane number with some loss in gasoline yield was shown in U.S. Pat. No. 3,758,403, the entire contents of which are incorporated herein by reference. In this patent, the cracking catalyst was comprised of a large pore size crystalline zeolite (pore size greater than 7 angstrom units) in admixture with ZSM-5 type zeolite, wherein the ratio of ZSM-5 type zeolite to large pore size crystalline zeolite was in the range of 1:10 to 3:1.
The use of ZSM-5 type zeolite in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934. The first two patents disclose the use of ZSM-5 type zeolite in amounts up to about 5 to 10 wt %; the third patent discloses the weight ratio of ZSM-5 type zeolite to large pore size crystalline zeolite in the range of 1:10 to 3:1.
The ZSM-5 type catalyst, especially virgin catalyst, has exceedingly high activity. Researchers have attempted to take advantage of the activity of fresh ZSM-5 catalyst by adding only small amounts of it to FCC catalyst. Typical of such work is U.S. Pat. No. 4,309,280, the entire contents of which is incorporated herein by reference. This patent teaches the addition of very small amounts of powdered ZSM-5 catalyst, characterized by a particle size less than 5 microns. This patent teaches that adding as little as 0.25 wt % ZSM-5 powder to the circulating catalyst inventory in an FCC unit would increase dry gas production by 50% (from 3.9 wt % dry gas to 6.0 wt %; see Example 6 in Table 2).
The criticality of using only minuscule amounts of a ZSM-5 type zeolite to achieve improved results with respect to octane number and overall yield has been shown in U.S. Pat. No. 4,368,114. In this patent, the use of only minuscule quantities of additive catalyst was shown to give the same beneficial results that were once thought obtainable only by adding much larger quantities of ZSM-5 type catalyst.
In order to reduce automobile exhaust emissions to meet federal and state pollution requirements, many automobile manufactures have equipped the exhaust systems of their vehicles with catalytic converters. These converters contain catalysts which are poisoned by tetraethyl lead. Since tetraethyl lead has been widely used to boost the octane number of gasoline, refiners now have to turn to alternate means to improve gasoline octane number.
The removal of lead from the gasoline pool and the resultant demand for unleaded gasoline has increased the value of octane. Further, as the distillate demand grows, both in the United States and Europe, refiners will tend to operate their catalytic crackers in modes which promote the formation of distillates as compared to gasoline. However, the operational conditions tend to decrease gasoline octane. Therefore, an increasing demand for high octane gasoline has resulted. While the prior art has shown that zeolites, and in particular ZSM-5, can increase gasoline octane in cracking units, its use will be restricted if a significant gasoline yield penalty and high gas make accompany the octane gain. In this situation, only refiners who have available gas handling and lower hydrocarbon upgrading capacities will find zeolites, such as ZSM-5, attractive in their cracking operations.
One method of increasing octane number is to raise the cracker reactor temperature. This method, however, is very limited, since many units are now operating at maximum temperatures due to metallurgical limitations. Raising the cracker reactor temperature also results in increased requirements for the gas plant (i.e., gas compressor and separator). Since most gas plants are now operating at maximum capacity, any increased load could not be tolerated by the present equipment.
As can well be appreciated from the foregoing, it would be extremely desirable to have a process which will provide high octane unleaded gasoline without undue sacrifice of gasoline plus distillate yield. It would be even more desirable if such results could be obtained in conjunction with an increase in operational flexibility and without undue use of expensive catalysts.
It is thus an object of the present invention to provide a process for increasing gasoline octane number without significant loss of gasoline plus distillate yield in a catalytic cracking unit.
Further, it is an object of the present invention to overcome the deficiencies of the prior art.
These and other objects are fulfilled by the present invention, which is disclosed below.